It is well-known to use high intensity, focused acoustic wave energy, such as ultrasonic waves (i.e., acoustic waves having a frequency greater than about 20 kilohertz) to generate thermal ablation energy for treating internal body tissue, such as tumors. It is also well-known to employ a tissue imaging system (e.g., MRI) in order to guide the delivery of such high intensity ultrasound energy, and to provide real-time feedback. One such image-guided focused ultrasound system is the Exablate® 2000 system manufactured and distributed by InSightec Ltd, located in Haifa, Israel. (www.insightec.com).
By way of illustration, FIG. 1 is a schematic representation of a simplified image-guided, focused ultrasound system 100 used to deliver thermal energy to a target tissue mass 104 in a patient 110. The system 100 employs an ultrasound transducer 102 to deliver an acoustic energy beam 112 generated by a large number of individual piezoelectric transducer elements 116 mounted on a distal (outward) facing surface 118 (shown in FIG. 2) of the transducer 102. The transducer 102 is geometrically shaped and positioned in order to focus the ultrasonic energy beam 112 at a three-dimensional focal zone located within the target tissue mass 104. While the illustrated transducer 102 has a spherical cap configuration, there are a variety of other geometric transducer designs that may be employed.
In particular, ultrasound is vibrational energy propagated as a mechanical wave through a target medium (e.g., body tissue). In the illustrated system 100, the individual transducer elements 116 collectively generate the mechanical wave (or “acoustic beam”) 112 by converting respective electronic drive signals received from a system controller 106 into mechanical motion. Wave energy transmitted from the individual elements collectively forms the acoustic energy beam 112 as it converges on the target tissue mass 104. Within the focal zone, the wave energy of the beam 112 is absorbed (attenuated) by the tissue, thereby generating heat and raising the temperature of the target tissue mass to a point where the tissue cells are killed (“ablated”). An imager (e.g., an MRI system) 114 is used to generate three-dimensional images of the target tissue region 104 both before, during and after the wave energy is delivered. For example, the images may be thermally sensitive, so that the actual thermal dosing boundaries (i.e., the geometric boundaries and thermal gradients) of the target tissue region may be monitored.
The transducer 102 may be focused at different locations within the target tissue region 104 by mechanical movement, including orientation of the transducer. Electronic “beam steering” may additionally or alternatively be used to change location of the focal zone by making corresponding changes in the attributes (e.g., phase, amplitude, frequency) and the individual transducer element drive signals. In a typical tumor ablation procedure, the transducer 102 delivers a series of discrete pulses of high intensity acoustic wave energy, each for a sufficient duration to generate tissue-destroying heat in a given focal zone. The energy pulses are sequentially focused at a number of differing focal zones located in close proximity to one another, until complete destruction (“ablation”) of the target tissue region 104 is achieved. Further information regarding image-guided focused ultrasound systems and their use for performing non-invasive tissue (e.g., tumor) ablation procedures may be found, for example, in U.S. Pat. Nos. 6,618,620, 6,582,381, and 6,506,154, each of which is hereby incorporated by reference.
FIG. 3 is an MRI image of the heat intensity distribution 125 caused by a nominal “sonication” (delivery of acoustic energy) of a target tissue area using a spherical cap transducer (such as transducer 102 in system 100 of FIGS. 1 and 2). The heat intensity distribution 125 is shaped by the interaction of the acoustic beam with the tissue, as well as the frequency, duration, and power (i.e., mechanical pressure) of the beam. More particularly, the wave energy converges as it propagates (from left to right in FIG. 3) through a “near field” region 119 to a focal zone 120, which tends to have an elongate, cylindrical shape. The conversation of wave energy to heat is most intense in the focal zone 120, due to the convergence (and collisions) of the individual waves, and can be generally equated with the tissue destruction, or “ablation” volume. Notably, some of the wave energy is absorbed in the near field region 119, especially in the area adjacent to the focal zone 120. A further portion of the wave energy passes through the focal zone and is absorbed in a “far-field” region 122, which refers generally to the tissue region located distally of the focal zone relative to the transducer. Although the waves that pass through (or are reflected from) the focal zone 120 tend to diverge in the far field region 122, to the extent any such far field energy absorption occurs in a concentrated area, it can result in undesirable and potentially harmful (and painful) heating and necrosis of otherwise healthy tissue.
As described in PCT publication WO 2003/097162, which is hereby incorporated by reference, it is possible to increase the effectiveness of thermal dosing of a target tissue region by delivering one or more relatively high pressure, short duration acoustic energy pulses to generate air bubbles in tissue located in the intended focal zone just prior to delivering a regular “ablation energy” wave pulse. The presence of the bubbles serves to increase the mechanical-to-thermal energy conversion in the tissue, which, along with the added reflection and scattering of the main acoustic beam, has the positive effect of reducing the overall amount of potentially detrimental far-field energy absorption. However, as shown in FIGS. 4 and 5, while the overall amount of far-field energy is decreased by the presence of the bubbles at the focal zone center, a far greater concentration of the remaining far field energy is concentrated along a central beam propagation axis 124, extending distally from a center focal plane 126 of the “enhanced ablation” focal zone 128. This thermal energy concentration has the appearance of a thermal “tail” 130 that tapers into a highly undesirable stick portion 132 (best seen in FIG. 5), thereby elongating the effective tissue ablation region 134, and resulting in an even higher temperature in a close-in portion of the far field region 136 than occurs during a non-bubble enhanced sonication.